Professor Robert Hill: Medical and Developmental Genetics

The Genetic Basis of Limb and Gut Congenital Malformations

Summary

Birth defects occur as errors in human development and appear as alterations in body form in the foetus. Of the genetic factors involved, it is estimated that there are well over 1700 different inherited human disorders that alter embryogenesis; the majority of these cause multiple defects in the foetus. An understanding of the basis of these birth defects requires an investigation into the nature of the genes responsible. Our goals are to identify genes that play a fundemental role in embryo genesis and when mutated cause human congenital abnormalities. Our studies fall along two main lines of investigation...

Limb Patterning and Long Range Regulation of Shh

The ZRS is a cis-regulatory element that controls the expression of Shh in the developing limb bud (Fig. 4). Shh expression is asymmetric and is specifically located in the posterior limb margin in a domain called the the zone of polarising activity (ZPA). This expression pattern is essential for regulating the number and identity of the skeletal elements that compose the fingers and toes. The ZRS is also an extreme example of a long-range regulator. In the human genome, the ZRS resides at a distance of 1Mb from the SHH gene and is located within the intron of another gene that itself has no role in limb development. The ZRS is highly conserved over a length of ~800bp and has an ancient role in development of appendages having recently been identified in the chondrichthyan fishes.

Human preaxial polydactyly (PPD) on chromosome 7q36 includes a broad range of digit abnormalities on the preaxial or anterior side of the hands and feet. Mouse models for PPD were instrumental in defining the developmental basis for this abnormality and showed that preaxial polydactyly results from ectopic expression of Shh at a site along the margin opposite the ZPA; i.e., at the anterior side of the limb bud. The data are compelling that PPD is a regulatory disorder resulting from mutations within the ZRS. The ZRS mutations culminate in misexpression of Shh generating an ectopic ZPA and consequently, the production of supernumerary digits.

Figure 3. Long Range Regulation of Shh by the ZRS

Projects

Mutations that cause misexpression of a potent developmental regulator is a unique pathogenetic mechanism. Understanding this mechanism will lead to both insights into the disease process and a better grasp of the normal regulatory machinery.

Projects include:

Genetic manipulation of the cis-regulator to investigate long range regulatory mechanisms, insulator mechanisms and regulator/promoter interactions.
Development of novel approaches to identify important regulatory proteins.
Locus-wide scans to identify modified histone components necessary for long range developmental expression
Analysis of the process that insulates surrounding genes from ZRS regulatory activity
Analysis of chromatin structure of the entire Shh regulatory locus

Lab Members

Current lab members involved in this work are:

Dr Laura Lettice
Dr Alison Hill
Carlo DeAngelis
Paul Devenney
Iain Williamson
Elaine Kinsella
Silvia Peluso
Eve Anderson
Fay Cooper

These fall into two parts:

Limb Asymmetry and Long Range Regulation of Shh (this page)
Left/Right Asymmetry

Purpose

Our aims are to investigate developmental processes that participate in organogenesis and understand how genetic inaccuracies lead to human congenital abnormalities. Our animal model system of choice is the mouse, chosen for its powerful genetics and close relationship to human.

Approach, Progress and Future Work

We investigate the embryonic mechanisms that organise the overall design of the developing organ systems. These mechanisms define the structures and their final arrangement in composing the embryonic anatomy. We study two developmental systems in which the initial developmental stages have one aspect in common; that is, that each must accurately regulate asymmetric organisation of anatomical structures.

Figure 1. Preaxial Polydactyly in Mammals

Patterning of the Limb

Limb abnormalities occur frequently during human development. We are interested in genes responsible for regulating the pattern of the limb skeletal elements. A number of mutants in mouse disrupt the regulation of the limb digit pattern resulting in the production of extra digits. We are investigating the molecular basis of a number of these. One mouse mutation called sasquatch (Ssq) is the counterpart to human preaxial polydactyly (PPD) on chromosome 7q36 (Fig. 1,2). We showed that the extra toe phenotype is a result of mutations in a regulatory element which controls sonic hedgehog expression. We are identifying other genes that interact to regulate this expression.